Micro-Electro-Mechanical System (MEMS) devices find applications in a variety of fields, such as communications, sensing, optics, micro-fluidics, and measurements of material properties. In the field of communications, variable MEMS capacitors are used in tunable RF filter circuits. The MEMS capacitors offer several advantages over solid state varactor diodes, including a higher on/off capacitance ratio and a higher Quality (Q) factor. In addition, MEMS devices offer greater linearity compared to their solid state counterparts, which reduces intermodulation products when used in Radio Frequency (RF) applications.
Many of these MEMS devices comprise a beam or microstructure suspended above a substrate by one or more supports. In a variable MEMS capacitor, for example, the beam may be suspended above a bottom electrode on the substrate to form a capacitor with the beam acting as the top electrode. The capacitance of the variable MEMS capacitor is varied by establishing an electrostatic force between the beam and the bottom electrode. The electrostatic force bends the beam relative to the bottom electrode, thereby changing the gap between the beam and the bottom electrode and, therefore, the capacitance of the MEMS capacitor.
A problem in MEMS devices is permanent adhesion of the beam or microstructure to the underlying substrate. This phenomenon is commonly referred to as “stiction”. Stiction typically occurs when a beam or other microstructure is brought into intimate contact with the substrate. Once in contact, adhesion forces, e.g., Van der Waals force and/or hydrogen bonding, on the surface of the substrate exceed the elastic restoring force of the beam. As a result, the beam remains stuck to the substrate, rendering the MEMS device unusable.
Stiction may occur during fabrication of a MEMS device. For example, stiction may occur when a sacrificial layer used to temporary support the beam during fabrication is removed to release the beam. The sacrificial layer is often removed using a wet enchant followed by a rinse with a liquid agent, e.g., deionized (DI) water and/or methanol. As the device dries, liquid agent trapped between the beam and the substrate may produce capillary forces sufficient to pull down the beam into intimate contact with the substrate. Stiction may also occur during operation of a MEMS device, sometimes referred to as “in-use” stiction. For example, stiction may occur in a MEMS capacitor when the electrostatic force used to vary the capacitance of the MEMS capacitor pulls down the beam into intimate contact with the substrate.
Several approaches have been developed to alleviate stiction. One approach is to provide a rough surface on the substrate underlying the beam to reduce adhesion forces between the beam and the substrate. This is commonly done by depositing a nitride layer onto the substrate and wet etching grain boundaries in the nitride layer to produce a rough surface. A drawback of this approach is that “pin-holes” sometimes form in the nitride layer, which can lead to premature breakdown of the device. Another approach is to provide “dimples” or small bumps on the bottom surface of the beam to prevent the beam from coming into intimate contact with the substrate. A drawback of this approach is that it involves a cumbersome fabrication process that requires a planarization step, which may not be feasible in some process flows. Yet another approach is to use hydrophobic monolayers known as self assembled monolayers or SAM coatings to make the beam hydrophobic. This reduces the likelihood that water droplets will form under the beam and pull the beam down into intimate contact with the substrate.